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Subject: Re: [PATCH RESEND v2 2/2] KVM: Pre-allocate 1 cpumask variable per
 cpu for both pv tlb and pv ipis
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nick Desaulniers <ndesaulniers@google.com>
References: <1581988104-16628-1-git-send-email-wanpengli@tencent.com>
 <1581988104-16628-2-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <a232d6bf-0a33-25a6-e76d-b197e677217b@redhat.com>
Date:   Fri, 28 Feb 2020 10:35:05 +0100
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On 18/02/20 02:08, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Nick Desaulniers Reported:
> 
>   When building with:
>   $ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
>   The following warning is observed:
>   arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
>   function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
>   static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
>   vector)
>               ^
>   Debugging with:
>   https://github.com/ClangBuiltLinux/frame-larger-than
>   via:
>   $ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
>     kvm_send_ipi_mask_allbutself
>   points to the stack allocated `struct cpumask newmask` in
>   `kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
>   potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
>   the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
>   8192, making a single instance of a `struct cpumask` 1024 B.
> 
> This patch fixes it by pre-allocate 1 cpumask variable per cpu and use it for 
> both pv tlb and pv ipis..
> 
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Acked-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
>  * remove '!alloc' check
>  * use new pv check helpers
> 
>  arch/x86/kernel/kvm.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 76ea8c4..377b224 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -432,6 +432,8 @@ static bool pv_tlb_flush_supported(void)
>  		kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
>  }
>  
> +static DEFINE_PER_CPU(cpumask_var_t, __pv_cpu_mask);
> +
>  #ifdef CONFIG_SMP
>  
>  static bool pv_ipi_supported(void)
> @@ -510,12 +512,12 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
>  static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>  {
>  	unsigned int this_cpu = smp_processor_id();
> -	struct cpumask new_mask;
> +	struct cpumask *new_mask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
>  	const struct cpumask *local_mask;
>  
> -	cpumask_copy(&new_mask, mask);
> -	cpumask_clear_cpu(this_cpu, &new_mask);
> -	local_mask = &new_mask;
> +	cpumask_copy(new_mask, mask);
> +	cpumask_clear_cpu(this_cpu, new_mask);
> +	local_mask = new_mask;
>  	__send_ipi_mask(local_mask, vector);
>  }
>  
> @@ -595,7 +597,6 @@ static void __init kvm_apf_trap_init(void)
>  	update_intr_gate(X86_TRAP_PF, async_page_fault);
>  }
>  
> -static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
>  
>  static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>  			const struct flush_tlb_info *info)
> @@ -603,7 +604,7 @@ static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>  	u8 state;
>  	int cpu;
>  	struct kvm_steal_time *src;
> -	struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
> +	struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
>  
>  	cpumask_copy(flushmask, cpumask);
>  	/*
> @@ -642,6 +643,7 @@ static void __init kvm_guest_init(void)
>  	if (pv_tlb_flush_supported()) {
>  		pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
>  		pv_ops.mmu.tlb_remove_table = tlb_remove_table;
> +		pr_info("KVM setup pv remote TLB flush\n");
>  	}
>  
>  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> @@ -748,24 +750,31 @@ static __init int activate_jump_labels(void)
>  }
>  arch_initcall(activate_jump_labels);
>  
> -static __init int kvm_setup_pv_tlb_flush(void)
> +static __init int kvm_alloc_cpumask(void)
>  {
>  	int cpu;
> +	bool alloc = false;
>  
>  	if (!kvm_para_available() || nopv)
>  		return 0;
>  
> -	if (pv_tlb_flush_supported()) {
> +	if (pv_tlb_flush_supported())
> +		alloc = true;
> +
> +#if defined(CONFIG_SMP)
> +	if (pv_ipi_supported())
> +		alloc = true;
> +#endif
> +
> +	if (alloc)
>  		for_each_possible_cpu(cpu) {
> -			zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
> +			zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
>  				GFP_KERNEL, cpu_to_node(cpu));
>  		}
> -		pr_info("KVM setup pv remote TLB flush\n");
> -	}
>  
>  	return 0;
>  }
> -arch_initcall(kvm_setup_pv_tlb_flush);
> +arch_initcall(kvm_alloc_cpumask);
>  
>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>  
> 

Queued now, thanks.

Paolo